retro fit window

i forgot to look on the outside of the house when i measured for a new window. the old aluminum window frame stays in and the new retro window is put in from the outside cause it's a flush fin are on the outside and then gets screwed on the inside sides and top.my problem is i measured on the inside of the house and didn't notice the brick 1 inch below the window sill on the outside. can i cut the fin 1/2 the width on the bottom.or am i going to cut the top aluminum frame out and 1 inch into the header.?

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retro fit window

It sounds like you're putting a new construction unit into an opening with the old frame in place. I don't know what a "retro" window is. Windows are either replacement or new construction.
What covers the nailing flanges? Is this a brick house? If it is, you can use masonary clips to hold the window in.
See if you can post a photo of both the new window and the opening it's going into.
Ron